Various types of brakes, such as electric, air, drum, and disk, have been developed for various types of vehicles and uses. Air brakes, for example, are commonly used on heavy-duty trucks. Drum brakes have been commonly used on other automobiles where such heavy loads are not involved, but federal laws requiring disk-type brakes on new cars have caused the disk-type brake to become more popular. Disk-type brakes are usually less subject to water fading and are typically more resistant to heat fading due to high speed or repeated stops than drum brakes. Also, disk-type brakes are capable of straight-line stops so that hard braking effort usually does not cause the automobile to pull to the right or to the left.
Although disk brakes have several advantages over drum brakes, problems have also been discovered during some uses of disk brakes. For example, when this type of brakes are used with cars enduring high speeds over a long period of time, such as with race cars, the brake pedal often develops a spongy feeling or brake fade which reduces the brakes' effectiveness and produces less than optimum braking action. The spongy feeling in the brake pedal is typically caused by boiling hydraulic fluid, water, or the like in and around the brake caliper. The boiling hydraulic fluid, in turn, is caused by the heat generated from the application of the brakes at high speeds.
Some attempts to solve this problem have included forcing air into the caliper area or circulating water through the caliper area to keep the fluid from boiling. These attempts, however, have failed because of the lack of understanding of the braking system operation.
Other attempts recognized that some of the brake fade is caused by the brake fluid remaining in the caliper. Therefore, attempts to solve the problem based on this recognition have arranged check valves either within or closely adjacent the disk brakes to allow fluid to circulate through the brake caliper. An example of such a disk brake may be seen in U.S. Pat. No. 4,799,575 by Kroniger titled "Hydraulically Controlled Disk Brake With Serial Fluid Flow." These and other attempts, however, likewise failed because they failed to recognize the need to provide a path for purging air from the brakes and the brake lines and failed to recognize the residual pressure problems related to such braking systems.
Thus, there is a need for a braking system that provides circulation of fluid through the brake caliper and purges the brake lines of air or gas therein.